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Bamas house has a load of 31,650K. He put in a 3 ton which probably gives out around 35K. Now in a few years it will lose capacity do to dirty coils outside and inside, and fin corrosion over the years, which could add up to atleast 10%, so he will then have around 31,500K, which is right on top of his load calculation. Plus, with young kids running in and out, social events, you will be fine.

Fast eddy in the same scenario installs a 2 ton which puts out around 23,000 btu and loses 10% over the years will only be giving you 20,700 btu, almost a WHOLE ton shy of the load analysis.

I do not think I want to be paying for the extra ton shy of cooling over 20 years.

One last thing eddy, don't you think you should find out what direction the house faces also, or does the west, south sun angle matter to you?

Sorry to get off track on your thread, but my purpose was well intended and complete now. I did not want you to take the advice of a bad contractor and believe that your bad contractor was doing the right thing. Once Eddy made his mistake, I knew the better contractors here would back me up with the facts. So, as is my regular custom, I went to bed at a reasonable hour, and awoke refreshed. I hope you have taken the concensus of my peers as proof you need to change contractors?

Now for Eddy;
Manual J does indeed oversize equipment. No doubt about that. However, I do not oversize equipment, because I do not use Manual J. Not being of a lazy nature myself, I actually do a real load calculation, based on real conditions. Over a 15 year period of being a contractor, I mis-sized one system because the insulating value on the drawings was not the actual installed insulation value. But my load calculation records kept me from having to eat a replacement, and once the builder was forced to upgrade the insulation, the homeowner was happy.

A Bristol Twin Single is a 50-50 two speed.

A two ton unit is nominally 30,000 Btuh, so my load calc was "more" than 2.5 tons, not less, as you stated.

Larger coils require lower airflow in order to dehumidify. You specified oversized coils with higher airflow. Larger coils with higher airflow actually reduce dehumidification. I have not only posted the formulas here, and the lab test results, but have designed the most energy efficient RTU in the world based on that data. If you go to any Kohl's department store newer than 4 years, you'll see 15 units on the roof of each store that are running less than 250 cfm/ton, and dehumidifying like crazy. But when some lame contractor who thinks 400 cfm/ton is "always" right changes the sheave (there is a note on the cabinet that says you can't change the sheave without written permission from Kohl's, but people do it) to get 400, the humidity in the store goes to maximum (greater than outdoor) within 24 hours.

You continue to demonstrate your ignorance of the science related to your chosen profession, yet you strut and crow around here as if you invented the concepts yourself. Homeowners who care enough to want a quality install have no way to know (over the Internet) you are not only ignorant, but willing to lie to prove yourself correct. I have just prevented one homeowner from making that mistake by "outing" you.

Here are the facts of the matter:

The design conditions, which you admitted not looking up, and proved your ignorance of, are 105ºF DB and 60% RH. These are 95% numbers, meaning if you correctly size to those numbers, you'll be undersized 5% of the time. The load calc I did was exact, and nearly two years of experimentation while living there proved my load calc was correct. I even gave you the important part of that experimentation, when I told you that three tons maintained 68ºF on a 107 degree day. Was 68 my design indoor temp? No. I told you earlier the indoor design temp was 72. So, why was the system set for less than 72? And why was I able to maintain 68 at 107? Because I was proving my load calc, and the unit was producing a nominal capacity of 36,450 BtuH. (That capacity was calorimetered by Bristol compressor company, with my coil, at my airflow, so don't accuse me of faking the rocket science. . .I have the documentation)

The reason the first guy oversized my unit was because the window area was less than "normal." With no windows facing north or south, and almost no windows facing east, my solar gain was nearly negligible (that's a big word; it means you can ignore things that are). That's why I suffered through a summer of crazy temp swings, wild humidity levels and cyclone quality airflow.

The reason I went with three tons instead of two and a half was because of the existing coils. There was no way to get the indoor coil broken into small enough circuits and replacing it was outside my budget. I divided it into two circuits, of 1.5 tons each. The existing blower would not turn down below 1600 cfm, and throttling it with restrictions created lots of noise. So I installed a variable speed motor and programmed my own constants to get the airflows I wanted. (being an OEM design engineer, I can get free samples of stuff like that for experimentation)

Now, if you still want to claim you "won," I'll give you some more data, but except for one guy who agrees with you, the consensus seems to be you are wrong. And, mathematically, socially, and economically, you are wrong.

One is, your remarks after the facts do not correspond your quote about how the unit runs.

If an oversized coils does not do more dehum. Then why do Engineers spec 4 and 5 row coils for dehum,Instead of three. 450 cfm per ton is a dew point of 56

One problem I have is that you did the load calc.And that in itself tells me that it is probably flawed.

You continually refer to this load calc and try to hide behind it.But yet you do not explain why your house can keep 67 on 107, and cycle.

You openly admit that you have micalculated and installed the improper equipment in someones home before. ( this should be a great source of embarassment for you )

I openly admit that I have never missized any piece of equipment.

The reality is I made a pretty good damn assessment of your home without the benefit of seeing it. I never said that I choose equipment over the internet or over the phone. Trust me if your home actually does need 2.5 rather than 2 ton I would have made that determination on site.

It is industry standard to undersize residential cooling to the lower half ton, and you probably know this but are unwilling to admit it.

You are outmanned here Bama, you are up against the best,you can't win.

Originally posted by coldbud Mr. BC,
Thank you sir for your input. Mr. Eddy thank you sir for your input.

No problem, I hope this helped you because I like to feel that I did the right thing for a customer. Even if I have to teach others a lesson along the way. In the end it is the customer that counts.And I in turn would like to thank BC for keeping it clean and professional.Rememeber BC you are never to old to learn.

Fat Eddy, you are wrong. It isn't necessary to over-size the evap coil to increase the efficiency or the de-humidification factor. Just ask Airman1, or go to his website, & you'll see why.

I'm with Bama on this one. Contractors like you give honorable contractors like Bama a bad name. And besides, doing a load calc proves to the HO that you care about their needs & that you're not lazy (as is obvious that YOU are lazy by the fact that you don't like to do load calcs).

As a manufacturers technical rep I had come across many "fat eddie" type of contractors. Usually decent, hard working guys who have compiled a who lot of wrong information from incorrect analysis on why HVAC equipment does what it does. The prime example here is the "larger evap coils do a better job at dehumidification", which has been properly noted; they don't.

Most times I get along with the "fat eddies" of the HVAC world because once they have been shown the error of their ways they accept it and become better HVAC professionals. Then there are those who will continue to argue because they are just too proud to admit they were wrong. This is what we have experienced on this thread and that is a shame. Fat eddie seems to be a dedicated contractor and hopefully he will put the knowledge of this thread to good use in the future even if he does refuse to accept that he is wrong.

By the way eddie, you lost me when you made the remark "this isn't rocket science". Statements like that show an absolute disrespect for the scientific nature of our industry and an unwillingness to learn and understand the functions of HVAC systems.

As usual, Bama has kept a level head even when being confronted with ridiculous perceptions of science and twisted data.

I am not a big fan of residential load calcs on change out systems. However, if a load calc is requested, it needs to be done. If for no other reason then to satisfy the needs of the customer.

I feel sorry for the homeowners on this topic. After reading all the posts, a novice/homeowner could believe that critically sized, multi-featured a/c units will provide 75^F, 50%RH with any weather condition. For the homeowner, get it in writing! When it's raining for a week and 65^-75^f outside, no a/c and no humidity control. Save your money on the full featured a/c. Go high SEER, an extra 1/2 ton a/c for setback/company, and a ventilating dehumidifier. Now 50%RH regardless of outside temperatures, even a/c off. A high SEER a/c setup right will provide humidity control with high a/c load. Low/no load the dehu takes over. We add dehus to a lot of top full featured a/cs because of no humidity control with low cooling load. If you want fresh air and humidity control with rainy cool weather, reheat or dehumidification is the only way. The load calc is good, but add .5 ton and a dehu.